Mem. S.A.It. Vol. 86, 362
SAIt 2015 c
MemoriedellaChemical analysis of a new kinematically identi ed stellar group
R. ˇZenovien˙e
1, G. Tautvaiˇsien˙e
1, B. Nordstr¨om
2, E. Stonkut˙e
11
Institute of Theoretical Physics and Astronomy, Vilnius University, A. Gostauto 12, 01108, Vilnius, Lithuania
e-mail: renata.zenoviene@tfai.vu.lt; grazina.tautvaisiene@tfai.vu.lt;
edita.stonkute@tfai.vu.lt
2
Dark Cosmology Centre, Niels Bohr Institute, Copenhagen University, Juliane Maries Vej 30, 2100, Copenhagen, Denmark
e-mail: birgitta@astro.ku.dk
Abstract.
We have started a study of chemical composition of a new kinematically iden- tified group of stars in the Galactic disc. Based on dynamical properties those stars were suspected to belong to a disrupted satellite. The main atmospheric parameters and chemical composition were determined for thirty-two stars from high resolution spectra obtained at the Nordic Optical Telescope with the spectrograph FIES. In this contribution the prelimi- nary results of chemical composition study are presented. The metallicity of the investigated stars lie in the interval –0.2 < [Fe/H] < –0.6, their abundances of oxygen and alpha-elements are overabundant in comparison to the Galactic thin disc dwarfs at this metallicity range.
This provides further evidences of their common and possibly extragalactic origin.
Key words.
Stars: abundances – Galaxy: evolution – Galaxy: disc – Galaxy: formation
1. Introduction
Ancient mergers of dwarf galaxies are believed to play an important role in the formation of the Milky Way galaxy. Evidences of small dwarf galaxy mergers to our Galaxy are the Sagittarius (Ibata et al. 1994) and Canis Major (Martin et al. 2004) dwarf irregular galaxies.
However, there is much harder to discern re- licts of past accretion events in the galactic disc, where spatial, kinematic and metallicity distributions overlap.
Helmi et al. (2006), using the F- and G- type dwarf star survey by Nordstr¨om et al.
(2004), identified three new coherent groups of stars. Simulations of conserved orbital param-
eters suggest that those stars might correspond to remains of disrupted satellites. Stars in those groups have distinct kinematics, metal- licity [Fe/H] and age distribution, providing evidences of their extragalactic origin.
According to Helmi et al., one of the newly identified groups (Group 2) contains 86 stars and shows evidences of three populations with ages of 8 Gyr (15 %), 12 Gyr (36 %) and 16 Gyr (49 %), respectively. We started to investi- gate the detailed chemical composition of stars in this group.
Detailed chemical abundances are the key tools to reveal the origin of such stellar groups.
Using the high resolution spectra we measure
ˇZenovien˙e: Chemical analysis of a new kinematically identified stellar group 363
[El/Fe]
O Mg Si
[El/Fe]
[Fe/H]
Ca
[Fe/H]
Ti
[Fe/H]
Ni
Fig. 1. [El/Fe] ratio as a function of [Fe/H] in the investigated stars suspected to belong to a disrupted satellite (filled circles). The data for the Milky Way disc dwarfs are taken from Edvardsson et al. (1993, plus sings), Bensby et al. (2005, stars), Reddy et al. (2006, squares), Zhang & Zhao (2006, triangles). The Galactic thin disc chemical evolution model is shown by the solid line (Pagel & Tautvaisiene 1995).
abundances of oxygen, α-elements and iron group elements and make a comparison with Galactic thin disc stars. Here we present the preliminary results for the stars of the Group 2.
All the details about the method of analysis can be found in Stonkut˙e et al. (2012).
2. Results and conclusions
Our investigations show that the sample of stars is chemically homogeneous (Fig. 1) which supports a common origin of those stars.
The averaged value of [Fe/H] for the stars in- vestigated in the Group 2 is −0.4 ±0.1 dex.
The iron-group element to iron abundances are close to solar while the mean abundances of oxygen and α-elements are overabundant in comparison to the Galactic thin disc dwarfs and the Galactic thin disc chemical evolution model by Pagel & Tautvaisiene (1995). The distinct chemical composition together with the kinematic properties give further evidences that the Group 2 stars might be of the extra- galactic origin.
Acknowledgements. The research leading to these
